A known indexing device is, for example, a device (rotary table) mounted on a bed of a machine tool and being capable of indexing a table on which a workpiece is mounted. For example, a rotary table disclosed in Patent Document 1 includes a frame having a through hole at the center of the frame, and a rotary shaft inserted into the through hole and supported rotatably relative to the frame. A table surface, on which a workpiece is mounted, of the rotary table is integrally provided with the rotary shaft. A worm wheel is accommodated within the frame, the work wheel being integral with the rotary table. A worm spindle is also accommodated within the frame at a position proximate to the worm wheel in a manner meshing with the worm wheel. The worm spindle is linked with an actuator such as a servomotor. When the servomotor is rotationally driven, the rotary table being integral with the rotary shaft rotates by a desired angle, thereby carrying out an indexing operation.
Meanwhile, the rotary table disclosed in Patent Document 1 includes a clamp mechanism (clamp sleeve) which retains an angle (position) of the rotary table after the indexing operation. In particular, the rotary table includes a cylinder segment extending in parallel to the through hole of the frame and integrally provided with the rotary table, and a clamp sleeve having a ring shape and provided in a space between the cylinder segment and the frame. The clamp sleeve includes a cylindrical portion fitted around the cylinder segment of the rotary table, and a flange portion continuously provided with the cylindrical portion and functioning as an attachment portion to the frame. The clamp sleeve has a thin-wall section provided outside the cylindrical portion and extending in an axial direction. A pressure chamber is formed between an inner space of the thin-wall section and the through hole of the frame accommodating the cylindrical portion. During a clamping operation, the thin-wall section expands inward in a radial direction by pressure oil supplied to the pressure chamber. Hence, a shaft portion of the rotary table with the angle thereof positioned can be held in a non-rotatable manner.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-103181